1. Field of the Invention
This invention relates generally to an improved system for purifying contaminated air, and more particularly to a falling bed adsorber for removing organic contaminants from exhaust air using adsorbent material, where a minimal amount of system air pressure is required.
2. Description of the Background Art
In the prior art, methods of removing Volatile Organic Compounds (VOCs) from contaminated air include fixed bed adsorbers and fluidized bed adsorbers. Fixed bed adsorbers are the most common method, and comprise a canister containing adsorbent particles of a polymer through which contaminated air is passed. The VOCs in the contaminated air are deposited onto the polymer by adsorption or absorption, or a combination of both. The air exiting the canister is largely devoid of the VOCs. One downfall of this type of system is that, eventually, the polymer in the canister becomes saturated with the VOCs removed from the contaminated air and must be replaced. Although most polymers can be reconditioned and reused, it is very costly to continually change canisters and recondition the polymer. Another problem with fixed bed adsorbers is that they are quite large due to the fact that the particles of adsorbent polymer in the bed contact each other, thereby reducing the exposed surface area of each particle that is available for contact with the VOCs. In addition, a great amount of system air pressure is required to force the contaminated air through the material within the canister.
Fluidized bed adsorbers comprise "sieve trays" or flat beds atop which the adsorbent particles are placed, where each bed has a plurality of holes in its base that allow the contaminated air to pass through without allowing the adsorbent particles to fall through the holes. Fluidized beds were created to distribute contaminated air through layers of adsorbent polymer in a more uniform way in an effort to increase efficiency. As the velocity of the contaminated air passing through the bed is increased, the adsorbent particles separate slightly and expose more of their surfaces to the air stream. At some point, the air velocity causes the adsorbent particles to act as a fluid, and causing bubbles to be formed in the air as the contaminated air passes through the bed. The problem with bubbles is that only the outer surface of the bubble is exposed to the adsorbent particles, and thus a reduced amount of VOCs are adsorbed onto the polymer and removed from the air stream. Another problem with fluidized bed systems is that two or more beds must be used so that the air bubbles created in the first bed are broken by the next bed in order to expose more of the contaminated air to the adsorbent particles.
Although the pressure required to pass the contaminated air through a series of fluidized beds is less than that required to pass contaminated air through a fixed bed adsorber, still a relatively large pressure drop is realized across the adsorbers in fluidized beds. In other words, a high amount of pressure is required to force the contaminated air through the holes in the sieve trays and through the layer of adsorbent material atop the sieve tray. Since higher system pressures require larger fans with more horsepower, this system is undesirably large and consumes a considerable amount of energy.
What is needed is a low pressure adsorbent system that efficiently mixes contaminated air with adsorbent material such that the full surface of the adsorbent material is exposed to the contaminated air, and a system that also reconditions the saturated adsorbent material and recycles it back into the system.